Assemblies employing micropipettes for delivering a precise known volume of fluid are in use, for example, Trenner pipette used to obtain an accurate blood sample to count blood cells. In practice, a rubber tube is fitted over the upper end of the pipette, one end of the rubber tube is held in the mouth and the lower end of the pipette is held in a drop of blood. The blood is sucked into the pipette to the mark, the end of the pipette is wiped off and if an excess amount of blood is in the pipette an absorbent material such as tissue paper is used to remove excess blood. A diluent is then drawn into the pipette to a second mark to dilute the blood sample. If there is an error in filling to either mark then the dilution is inaccurate which results in erroneous test results.
Another method employed is to fill a micropipette to a fill mark indicated thereon and hold the pipette substantially horizontally and intentionally overflow the pipette. Then an absorbent tissue is applied to the wet end of the pipette and by capillarity a portion of the liquid is removed from the pipette. However, the miniscus of the liquid column is hard to control in this fashion since overfill or underfill may easily occur.
Conventional micropipettes of the type having capillary bores to be filled to a mark are difficult to handle and to measure accurately precise volumes of a fluid, it requires considerable experience and dexterity by a technician and, in many cases, leads to error in determining the test results.
Another prior art method for accurately filling a capillary with a first liquid, such as blood, serum, plasma, or chemical solution or mixture, and then mixing it with a predetermined volume of a second liquid, such as a diluent, or reagent, or liquid mixture, is commercially available and is sold under the trade name UNOPETTE System and is described in U.S. Pat. Nos. 3,045,494, 3,433,712 and others. In these patents the capillary is a tube which automatically fills throughout its length and by controlling the bore diameter and tube length a predetermined volume of liquid fill is achieved with high accuracy.
Capillary tubes automatically fill due to several factors. The tube is made of a material, usually glass, which has high wettability characteristics relative to the liquid, such as blood, throughout its length. Very small bore diameters are used whereby the surface tension forces are large, tending to draw the liquid into the tube. Also, the meniscus formed at each end of the liquid column is fairly strong relative to the weight of the liquid column, so that the liquid-filled tube is comparatively immune to the flowing out at either end due to tipping or other handling, at least to a practical extent in normal careful use.
When using capillary tubes care is taken to be sure that neither liquid-filled end is touched to any nearby object, because a drop of blood might then be transferred to such object and the volumetric accuracy of the remaining blood would be lost and the test would be ruined.
There has been a standing need for additional kinds of tests to utilize the UNOPETTE System for chemical and blood tests where the original sample, the first liquid, should be much larger in volume than heretofore -- as much as 100 percent larger, even 200 percent or 300 percent or more. It is impractical to merely increase the length of the glass capillary by such large amounts because they would be so fragile and so frequently broken as to be prohibitive. The bore size needs to be radically increased. However, with each increment of increase in bore size, the capillary forces decrease, and more than proportionally. Also the forces at the meniscus at each end rapidly diminish while the weight of the liquid column increases more than proportionally relative to the bore diameter. These factors combine to make a filled capillary of substantially increased bore size so extremely sensitive to tipping (so that liquid runs out the lower end) that it is impractical.